1. Field of the Invention
The present invention relates to a knitting yarn for a gland packing to be used at a shaft sealing portion of a fluid device such as a pump, and also relates to a gland packing made of such knitting yarn.
2. Description of the Prior Art
The following packings are known as gland packings of the type mentioned above.
One packing is made by spirally or concentrically winding a tape-like expanded graphite having an excellent compression-restoring force and sealing properties, and pressing and molding, in molds, the expanded graphite thus wound, thereby to form a ring-like gland packing of the die-mold type. Another packing is made by punching expanded graphite into ring-like pieces and laminating and molding a plurality of ring-like pieces thus punched, thereby forming a gland packing of the laminate type.
Out of such conventional gland packings, the packing of the die-mold type can be applied to a shaft having a predetermined diameter but cannot be applied to shafts having different diameters. Thus, such a packing lacks versatility. It is therefore required to previously prepare a variety of packings having different diameters in order to deal with shafts having different diameters. Further, expanded graphite itself is poor in tensile strength, and is therefore fragile. Accordingly, when it becomes necessary to replace such a packing mounted in the packing box, it is difficult to take out the packing from the packing box, thus presenting a problem in view of practical utility.
Further, such a gland packing is arranged such that layers are formed in the radial direction. This tends to produce an axial slip of the layers between adjacent layers. This causes portions of the packing to protrude into gaps, for example, between the shaft and the packing box, between the shaft and the packing gland and between the packing box and the packing gland. This reduces the volume of the packing and releases the applied stress, causing the sealing properties to be lowered. This not only increases the amount of leakage of a fluid, but also provokes the permeation and leakage of the fluid from the layers.
On the other hand, the gland packing of the laminate type is arranged such that layers are formed in the axial direction. This prevents portions of the packing from protruding into the gaps due to axial slip as done in the gland packing of the die-mold type. This advantageously restrains the sealing properties from being lowered because of the release of stress due to reduction in volume. However, the gland packing of the laminate type presents the problems of poor versatility for different shaft diameters and difficulty in taking out such a gland packing from the packing box for replacement, etc., likewise the packing of the die-mold type. Further, the gland packing of the laminate type inevitably produces a great amount of material loss, thus increasing the production cost.
The problems mentioned above be solved by proposing a braided packing with the use of expanded graphite which is braided or twisted such that the expanded graphite can be used as cut according to the diameter of a shaft, as done in a conventional braided packing.
The expanded graphite itself is a particle (powder) having a green-caterpillar-like appearance as expanded in the C-axis direction of a graphite particle crystal. The expanded graphite is poor in tensile strength and is fragile. Accordingly, the expanded graphite alone cannot be used as a knitting yarn to be braided or twisted.
In this connection, a variety of proposals have been made such that expanded graphite is used as a knitting yarn to be braided or twisted.
In FIG. 12 showing one such proposal, a knitting yarn 14 is made of a string-like body 13 having a composite structure which comprises an expanded graphite tape 11 so cut as to have a narrow width and a yarn-like reinforcing member 12, the expanded graphite tape 11 being spirally wound on the outer periphery of the yarn-like reinforcing member 12. In FIG. 13, showing another proposal, a knitting yarn 24 has a composite structure which comprises (i) a string-like body 13 in which a plurality of narrow-width expanded graphite tapes 11 are laminated, and (ii) a reinforcing braided member 15 made by knitting or circular-knitting fibers, the reinforcing braided member 15 covering the entire outer periphery of the string-like body 13. In FIG. 14, showing a further proposal, a knitting yarn 34 has a composite structure which comprises (i) a string-like body 13 in which a plurality of narrow-width expanded graphite tapes 11 are laminated, (ii) a plurality of yarn-like reinforcing members 16 interposed between adjacent layers in the longitudinal direction thereof, and (iii) a reinforcing braided member 15 made by knitting or circular-knitting fibers, the reinforcing braided member 15 covering the entire outer periphery of the string-like body 13. In FIG. 15, showing still another proposal, a knitting yarn 44 has a composite structure comprising (i) a plurality of yarn-like reinforcing members 16 and (ii) green-caterpillar-like expanded graphite particles 11B integrally bonded to both surfaces of the yarn-like reinforcing members 16.
Out of the four different knitting yarns using expanded graphite, the knitting yarn 14 in FIG. 12 is made by cutting an expanded graphite sheet into the expanded graphite tape 11 having a narrow width. Cutting the expanded graphite sheet into narrow-width tapes is itself very difficult. Further, the knitting yarn 14 is made by merely spirally winding the narrow-width expanded graphite tape 11 on the outer periphery of the yarn-like reinforcing member 12 such that the tape 11 covers the yarn-like reinforcing member 12. Accordingly, the integrally bonding force between the expanded graphite tape 11 and the yarn-like reinforcing member 12, is small so that the knitting yarn 14 is poor in shape retention. Thus, the tape 11 and the yarn-like reinforcing member 12 are liable to be separated from each other. When the knitting yarn 14 is poor in shape retention, it is difficult to braid or twist the knitting yarn 14 to produce a gland packing. Also, the resulting gland packing itself is poor in shape retention, thus lowering its sealing performance as a packing.
In the knitting yarn 24 in FIG. 13, the integrally bonding force between the string-like body 13 made of the expanded graphite tapes 11 and the reinforcing braided member 15, is strong and therefore superior in shape retention to the knitting yarn 14 in FIG. 12. This restrains the string-like body 13 and the member 15 from being separated from each other. However, cutting an expanded graphite sheet into narrow-width tapes 11 is itself very difficult likewise in the knitting yarn 14 in FIG. 12. Further, the knitting yarn 24 in FIG. 13 is liable to present relative movement or interlaminar slip among the layers of the string-like body 13 in which the plurality of expanded graphite tapes 11 are laminated. When forming a gland packing from the knitting yarns 24 comprising the string-like body 13, which is liable to present such interlaminar slip, the expanded graphite tapes 11 are liable to be broken when braiding or twisting the knitting yarns 24. This makes braiding or twisting difficult, preventing the resultant gland packing from being provided with a high sealing performance.
Likewise the knitting yarn 24 in FIG. 13. The knitting yarn 34 in FIG. 14 has a strong integrally bonding force between the string-like body 13 made of the expanded graphite tapes 11 and the reinforcing braided member 15. This not only produces an excellent shape retention, but also restrains the body 13 and the member 15 from being separated from each other. Further, the joint use of the yarn-like reinforcing members 16 further enhances strength. However, the knitting yarn 34 presents the problem that cutting an expanded graphite sheet into the narrow-width expanded graphite tapes 11 is itself very difficult and that the string-like body 13 is liable to present interlaminar slip, as in the knitting yarn 24 shown in FIG. 13. In addition, the interlaminar slip weakens the integrally bonding force between the string-like body 13 and the yarn-like reinforcing members 16, so that the yarn-like reinforcing members 16 are liable to be unevenly distributed. This injures uniformity of strength. With the use of the knitting yarn 34 presenting such interlaminar slip and uneven distribution of the yarn-like reinforcing members 16, the expanded graphite tapes 11 are liable to be broken when braiding or twisting the knitting yarn 34 in order to produce a gland packing. Further, the uneven strength prevents the sealing performance of the packing from being sufficiently increased.
In the knitting yarn 44 in FIG. 15, it is difficult to arrange expanded graphite particles in high density, as well as to manufacture large-size particles of expanded graphite. Further, the plurality of yarn-like reinforcing members 16 are liable to be separated from one another, thus preventing the sealing performance of the packing from being enhanced.